7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Reconsideration of ELMs
Kimitaka ItohChubu University
e-mail: <itoh@isc.chubu.ac.jp>
The Edge Localized Modes (ELMs) have been
observed in the H-mode [1]. The type-I ELMs could bea substantial obstacle in realizing the fusion reactor.
Classification of ELMs was given phenomenologically,
and is not necessarily based on the mechanisms. The
ELM bursts have been conventionally imagined as MHD
activiries (except ‘dither’, which is considered as limit
cycles of turbulence and meso-mean plasma parameters[2,3]). A majority line of thought on the type-I ELMs
employs the peeling-ballooning (P-B) mode as a central
player (See, e.g., a review [4]). Nevertheless, a few
essential problems remain, i.e., the identification of P-Bmode in the trigger, the reason why the so-called‘resonant magnetic (RM) field’ [5] is effective insuppression, etc.
Driven by these motivations, precise studies of trigger
mechanisms have been made on type-III ELMs (HL-2Aplasma) [6] and on type-I ELMs (JT-60U plasma) [7].
Here we report new discoveries that could rewrite theconventional picture of trigger mechanisms of ELMs.
The first example is the study of type-III ELM onHL-2A. As was reported in [6], the experimental
measurement (by use of the Langumuir probe) was madein the region of transport barrier of the H-mode. Before
the onset of ELM bursts, fluctuations at several tens ofkHz emerge, and then turn into the streamer. The
streamer is also observed on the ECEI data, and extend
into the core region with the perturbation of the electrontemperature (which is a few % of the mean temperature).
At the sudden onset of the streamer, the burst of plasmaenergy occurs. The magnitude of the streamer wasstrong enough to induce the observed loss of energy [8].
This study rewrites the picture of the type-III ELMs,
which has been thought to be results of some kind of
MHD instabilities. Rather, (at least in the case that wasstudied) it is a transition from a localized strong radial
electric field structure to the streamer. In the transportbarrier, the strong radial electric field induces poloidaland toroidal flows [9]. Both flows are inhomogeneous:while the former can suppress gradient-driven turbulence
so as to establish transport barrier, the latter can drive
parallel velocity gradient (PVG) instability. The PVD
turbulence can generate a streamer. One can propose
the transition from zonal flow to streamer as a newmechanism that triggers the onset of type-III ELM.
The next example is the detailed study of type-I ELM
on JT-60U [7]. As a basis for this study, Kamiya andcollaborators have succeeded in identifying the P-B
mode in the H-mode plasma [10]. The P-B mode wasidentified in the edge-harmonic oscillation, andconfirmed predicted characteristics (medium poloidalmode number, parity of displacement, etc.). Thisobservation was employed as a touchstone to identify

what happens in the triggering of the type-I burst in
JT-60U. As has been ubiquitously observed in ‘abrupt
events’ in laboratory and astrophysical plasmas, in theinstance of triggering bursts, ‘the abrupt increment of thegrowth rate’ was also observed in the type-I ELMs.
The property of the trigger mode was identified
unambiguously that the poloidal and toroidal mode
numbers are (m, n) = (4,1). This implies that the mode
has a resonance surface in the transport barrier region,and that it could be the tearing mode (not the P-B mode).
The tearing parity of the temperature perturbation wasconfirmed in the precursor phase and until the real
trigger happens. In addition, the strong nonthermal
emission in the ECE is detected in the phase of rapid
growth of the (4,1) mode, suggesting a fast reconnection.
The mechanism of trigger was studied. The onset
condition of the magnetic stochasticity was analyzed
based on the observed magnitude of the (4,1) mode, andwas found to agree, semi-quantitatively, with the onset
time of the trigger. If magnetic stochasticity takes place,the anomalous current diffusivity is switched-on, andexplosive growth can happen [11]. The time scale ofthe explosive growth also shows semi-quantitative
agreements with measurements.
These two examples urge us to reconsider the trigger
mechanisms of ELMs. Results here provide aperspective to classify ELMs based on the triggering
mechanisms, as well as a hope to formulate a dependable
guideline to control them.
Acknowledgements:
Major part of the presented novel work was performedin collaboration with Dr. K. Kamiya (QST), whom theauthor cordially acknowledges. Collaboration with Drs.
J. Cheng, J. Q. Dong, K. J. Zhao is also acknowledged.
This work is partly supported by KAKENHI 21K03513
and 20K03913. This work is dedicated to the memoryof late Prof. Sanae-I. Itoh.
References:[1] ASDEX Team: Nucl. Fusion 29 (1989) 1959
[2] S.-I. Itoh, et al.: Phys. Rev. Lett. 67 (1991) 2485
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